Miving picture encoding method and moving picture decoding method

ABSTRACT

A moving picture coding apparatus  1  includes a counter unit  102  which counts the number of pictures following an intra coded picture; and a motion estimation unit  101  which compares respectively only reference pictures which are the intra coded picture or the following pictures, selected from among a reference picture Ref 1,  a reference picture Ref 2  and a reference picture Ref 3  stored in memories  408˜410 , with a picture signal Vin, and determines the reference picture whose inter picture differential value is smallest.

TECHNICAL FIELD

[0001] The present invention relates to a moving picture coding method,a moving picture coding apparatus, a moving picture decoding method anda moving picture decoding apparatus for coding or decoding a movingpicture signal with reference to a plurality of coded pictures ordecoded pictures, and to a recording medium which stores a program forexecuting such coding or decoding in the form of software.

BACKGROUND ART

[0002] In the age of multimedia which integrally handles audio, videoand other information such as pixel values, existing information media,i.e., newspapers, magazines, televisions, radios, telephones and othermeans through which information is conveyed to people, have recentlycome to be included in the scope of multimedia. Generally, multimediarefers to something that is represented by associating not onlycharacters, but also graphics, voices, and especially pictures and thelike together, but in order to include the aforementioned existinginformation media in the scope of multimedia, it appears as aprerequisite to represent such information in digital form.

[0003] However, when calculating the amount of information contained ineach of the aforementioned information media as the amount of digitalinformation, while the amount of information per character is 1˜2 bytes,the amount of information to be required for voice is 64 Kbits or overper second (telephone quality), and 100 Mbits or over per second formoving pictures (current television reception quality), and it is notrealistic for the aforementioned information media to handle such anenormous amount of information as it is in digital form. For example,although video phones are already in the actual use via IntegratedServices Digital Network (ISDN) which offers a transmission speed of 64Kbps˜1.5 Mbps, it is not practical to transmit video information shot bytelevision cameras directly through ISDN.

[0004] Against this backdrop, information compression techniques havebecome required, and moving picture compression techniques compliantwith H.261 and H.263 standards recommended by ITU-T (InternationalTelecommunication Union—Telecommunication Standardization Sector) areemployed for video phones, for example. Moreover, according toinformation compression techniques compliant with the MPEG-1 standard,it is possible to store picture information in an ordinary music CD(compact disc) together with sound information.

[0005] Here, MPEG (Moving Picture Experts Group) is an internationalstandard on digital compression of moving picture signals, and MPEG-1 isa standard for compressing television signal information approximatelyinto one hundredth so that moving picture signals can be transmitted ata rate of 1.5 Mbps. Furthermore, since transmission speed within thescope of the MPEG-1 standard is limited primarily to about 1.5 Mbps,MPEG-2, which was standardized with a view to satisfy requirements forfurther improved picture quality, allows data transmission of movingpicture signals at a rate of 215 Mbps.

[0006] Furthermore, MPEG-4 which provides a higher compression ratio hasbeen standardized by the working group (ISO/IEC JTC1/SC29/WG11) whichwas engaged in the standardization of MPEG-1 and MPEG-2. Not only is itpossible to perform a highly efficient coding at a low bit rate, MPEG-4employs a powerful technique for error resilience which lessens thedegradation of picture quality to be judged from a subjectivestandpoint, even when a transmission channel error occurs. Also, ITU-Thas started work for standardization of H.26L as a next-generationpicture coding method.

[0007] MPEG-1, MPEG-2 and MPEG-4 have allowed substantial improvement ofcompression ratio using inter predictive picture coding (hereinafterreferred to as inter picture coding) for coding or decoding adifferential value between a current picture to be coded or decoded anda reference picture (a picture signal of a picture which has been codedor decoded most recently) with reference to the reference picture (See,for example, ISO/IEC 13818-2 “INTERNATIONAL STANDARD Informationtechnology Generic coding of moving pictures and associated audioinformation: Video”, Dec. 15, 2000, p. 7, Intro. 4.1.1).

[0008] In addition, compression of the amount of information can berealized by reducing redundancies in the temporal and spatialdirections. In the inter predictive picture coding which aims atreducing the temporal redundancies, a predictive picture is created withreference to previously coded or decoded pictures (reference pictures)and the differential value between the resulting predictive picture anda current picture to be coded is coded. Here, a picture is a termrepresenting one sheet of an image, and specifically, a picture means aframe in a progressive image and a frame or a field in an interlaceimage.

[0009] As of September, 2001, the proposed H.26L standard allows notonly reference only to a picture which has been coded or decodedimmediately before a current picture to be coded or decoded, but alsoreference to an arbitrary picture selected, as a reference picture, froma plurality of pictures which have been coded or decoded prior to thecurrent picture.

[0010]FIG. 1 shows an illustration of a concept of a conventional movingpicture coding method and a moving picture decoding method. FIG. 1 is anexample in which an arbitrary picture is selected as a reference picturefrom 3 pictures preceding a current picture to be coded or decoded. InFIG. 1, pictures are ordered in display order, and the display time ofthe picture at the far left is earliest. The pictures are also coded inthis order from the left. Therefore, in a bit stream, the pictures arealso ordered in the order of Picture J₁, Picture J₂, Picture J₃ andPicture J₄. When a current picture to be coded or decoded is Picture J₄,it is possible to select one of these Picture J₁, Picture J₂ and PictureJ₃ as a reference picture, and when a current picture to be coded ordecoded is Picture J₅, it is possible to select one of these Picture J₂,Picture J₃ and Picture J₄ as a reference picture.

[0011]FIG. 2 is a block diagram showing the structure of a conventionalmoving picture coding apparatus.

[0012] A moving picture coding apparatus 4 is an apparatus forcompressing and coding an inputted picture signal Vin so as to output acoded picture signal Str in the form of a bit stream transformed byvariable length coding or the like, and is comprised of a motionestimation unit 401, a selection unit 402, a picture signal subtractionunit 403, a coding unit 404, a decoding unit 405, an addition unit 406,a selection unit 407 and memories 408˜410.

[0013] The motion estimation unit 401 reads out previously codedreference pictures which are stored in the memories 408˜410 respectivelyand compare them with the inputted picture signal Vin so as to determinemotion information MV indicating the reference picture Ref whose interpicture differential value (error energy) is smaller and the pixellocation for making the inter picture differential value smaller. Thereference picture Ref and the pixel location for making the error energysmallest is usually determined, but recently a method has been used fordetermining the motion information MV not just for making the errorenergy smallest but for making the error energy smaller and making thecompression ratio larger. Note that the information of the referencepicture Ref and the pixel location is hereinafter referred to as motioninformation MV collectively. The selection unit 402 outputs thereference picture selected from among a reference picture Ref1, areference picture Ref2 and a reference picture Ref3 which are stored inthe memories 408˜410, based on a reference picture instruction signalRefForm that is a switching instruction signal. The subtraction unit 403calculates a differential picture signal Dif between a picture signalVin and a reference picture Ref.

[0014] The coding unit 404 codes the differential picture signal Dif andthe motion information MV that is the information for identifying areference picture. The decoding unit 405 decodes coded data Coded whichhas been coded by the coding unit 404 to obtain a reconstructeddifferential picture signal RecDif. The addition unit 406 adds thereference picture Ref and the reconstructed differential picture signalRecDif. The selection unit 407 outputs the inputted decoded picturesignal Recon to any of the memories 408˜410 as a decoded picture signalRec1, a decoded picture signal Rec2 or a decoded picture signal Rec3, soas to enable the decoded picture signal to be referred to for coding thefollowing pictures.

[0015] Next, the operation of the moving picture coding apparatusstructured as above will be explained.

[0016] The picture signal Vin is inputted to the picture signalsubtraction unit 403 and the motion estimation unit 401. The motionestimation unit 401 reads out the reference picture Ref1, the referencepicture Ref2 and the reference picture Ref3 which are previously decodedpictures stored in the memories 408˜410, compares them with the inputtedpicture signal Vin so as to determine a reference picture whose interpicture differential value is smallest, and outputs the motioninformation MV that is the information for identifying the referencepicture and the pixel location to be referred to.

[0017] At the same time, the motion estimation unit 401 outputs areference picture instruction signal RefForm that is a switchinginstruction signal so that the selection unit 402 can select a referencepicture corresponding to the motion information MV and output it as areference picture Ref. Note that since a scene change or the like causesa loss of correlation between the pictures, the compression ratio of aninter coded picture could decline more than an intra coded picture (oran intra picture) which can be reconstructed only with a coded picturesignal of a picture to be coded. In this case, the motion estimationunit 401 indicates the intra picture coding by the motion information MVand outputs a reference picture instruction signal RefForm foroutputting a reference picture Ref4 which always outputs a value 0 as areference picture Ref. Note that the value of the reference picture Ref4does not always need to be 0 and may be an average value 128 in the caseof a luminance signal or an RGB color signal whose value is 0˜255, forexample.

[0018] Also, in order to prevent error propagation or enablereproduction to start from a picture at some midpoint in a coded picturesignal, a picture in every predetermined number of pictures needs to beintra coded so as to be reconstructed only with a coded picture signalof a picture to be coded. So, the motion estimation unit 401 can switchinto the intra picture coding forcibly according to the instruction ofan intra picture coding instruction signal Reset given from outside.

[0019] On the other hand, the subtraction unit 403 calculates thedifference between this picture signal Vin and the reference picture Refselected by the selection unit 402, and outputs the differential picturesignal Dif to the coding unit 404. Next, the coding unit 404 codes thedifferential picture signal Dif and the motion information MV outputtedfrom the motion estimation unit 401, and outputs the coded picturesignal Str and the coded data Coded. Here, the coded data Coded is datanecessary for reconstructing a picture, and the coded picture signal Stris a bit stream of the coded data Coded transformed by variable lengthcoding or the like.

[0020] The decoding unit 405 decodes the coded data Coded and outputsthe reconstructed differential picture signal RecDif to the additionunit 406. The addition unit 406 adds the reconstructed differentialpicture signal RecDif and the reference picture Ref selected by theselection unit 402, and outputs the decoded picture signal Recon to theselection unit 407. The selection unit 407 outputs the decoded picturesignal Recon to any of the memories 408˜410 as a decoded picture signalRec1, a decoded picture signal Rec2 or a decoded picture signal Rec3 sothat the decoded picture signal Recon can be referred to as a referencepicture for coding the following pictures. In this example, theselection unit 407 switches the memories so that the picture which hasbeen stored in any of these memories at the earliest time is overwrittenby a new decoded picture signal Recon.

[0021]FIG. 3 is a block diagram showing the structure of a conventionalmoving picture decoding apparatus.

[0022] A moving picture decoding apparatus 5 is an apparatus fordecoding a coded picture signal Str which has been coded by the movingpicture coding apparatus 4.

[0023] A decoding unit 501 decodes the inputted coded picture signal Strand outputs a reconstructed differential picture signal RecDif andmotion information MV. A motion reconstruction unit 502 decodes themotion information MV and outputs a reference picture instruction signalRefForm. Operations of a selection unit 503, a selection unit 505 andmemories 506˜508 are same as those of the selection unit 402, theselection unit 407 and the memories 408˜410 of the moving picture codingapparatus 4 as shown in FIG. 2. The addition unit 504 adds thereconstructed differential picture signal RecDif and the referencepicture Ref to output a decoded picture signal Vout (which correspondsto a decoded picture signal Recon in FIG. 2).

[0024] Note that in the above-mentioned moving picture coding apparatus4 and the moving picture decoding apparatus 5, motion compensation unitsnot shown in the figures are provided on the output sides of theselection unit 402 and the selection unit 503, respectively, and performmotion compensation for generating pixel values with pixel locationprecision in decimal degree which are pixel values with ½ pixel locationprecision or the like for interpolating the pixel values of thereference picture outputted from the memory.

[0025] By the way, in the above-mentioned conventional moving picturecoding apparatus and moving picture decoding apparatus, it is notdistinguished at all whether the reference picture is an intra codedpicture or one of inter coded pictures following the intra codedpicture. For example, in the illustration of FIG. 1 explaining theconcept of the conventional moving picture coding method and movingpicture decoding method, Picture J₂ is an intra coded picture andPicture J₁, Picture J₃, Picture J₄ and Picture J₅ are inter codedpictures, but Picture J₁ can be referred to as a reference picture forPicture J₄. If Picture J₄ refers to Picture J₁ as a reference picture,it means that Picture J₄ refers to, as a reference picture, the picturepreceding the intra coded Picture J₂However, when starting reproductionfrom a picture at some midpoint, for example, when starting decoding andreproduction from the intra coded Picture J₂ at a midpoint in a codedpicture signal, the decoding of Picture J₄ requires reference to thedecoded Picture J ₁. Therefore, a problem occurs that the picturesfollowing Picture J₄ cannot be correctly decoded.

[0026] Also, for example, if a stream error occurs at a midpoint in acoded picture signal and Picture J₁ cannot be correctly decoded due tothe error, a problem occurs that the pictures following Picture J₄cannotbe correctly decoded because the decoding of Picture J₄ requiresreference to Picture J₁, although the intra coded Picture J₂ can becorrectly decoded.

[0027] And so, the present invention has been conceived in view of theabove-mentioned circumstances, and aims at providing a moving picturecoding method, a moving picture decoding method and the like for makingit possible to start reproduction from an intra coded picture at amidpoint in a coded picture signal and thus reproduce the picturesfollowing the intra coded picture without error even if a stream errormay occur.

DISCLOSURE OF INVENTION

[0028] The moving picture coding method according to the presentinvention is a method for coding a moving picture on a picture bypicture basis, the moving picture coding method comprising: adetermining step for determining, as a reference picture, one of anintra coded picture and pictures which have been coded following theintra coded picture, from among a plurality of pictures stored in amemory; a coding step for reading out the determined reference picturefrom the memory, calculating a differential picture signal that is adifference between the read-out reference picture and a picture to becoded, and coding the calculated differential picture signal; and astoring step for decoding the coded differential picture signal, addingthe decoded differential picture signal with a picture signal of thereference picture, and storing a resulting picture in the memory.

[0029] Also, the moving picture coding method according to the presentinvention is a method for coding a moving picture on a picture bypicture basis, the moving picture coding method comprising: a specifyingstep for specifying an intra coded picture of a plurality of picturesstored in a memory as a picture which is an anchor for restricting acandidate for a reference picture; and a coding step for coding a signalindicating the picture which is an anchor for restricting a candidatefor a reference picture so that a picture to be coded following thepicture specified in the specifying step refers only to said specifiedpicture or pictures which have been coded following said specifiedpicture.

[0030] In addition, the moving picture coding method according to thepresent invention is a method for coding a moving picture on a pictureby picture basis, the moving picture coding method comprising: aspecifying step for specifying an intra coded picture of a plurality ofpictures stored in a memory as a picture which is an anchor forrestricting a candidate for a reference picture; a step for deleting apicture which has been coded prior to the picture specified in thespecifying step; and a coding step for coding a signal indicating thatthe picture which has been coded prior to the picture specified in thespecifying step is deleted so that a picture to be coded following saidspecified picture refers only to said specified picture or pictureswhich have been coded following said specified picture.

[0031] Further, the moving picture decoding method according to thepresent invention is a method for decoding a moving picture on a pictureby picture basis, the moving picture decoding method comprising: adecoding step for decoding an inputted coded picture signal; adetermining step for determining, as a reference picture, one of anintra decoded picture and pictures which have been decoded following theintra decoded picture, from among a plurality of pictures stored in amemory; and a storing step for reading out the determined referencepicture from the memory, adding a picture signal of the read-outreference picture with a decoded differential picture signal of apicture to be decoded, and outputting a resulting picture to the outsideand storing the resulting picture in the memory.

[0032] Also, the moving picture decoding method according to the presentinvention is a method for decoding a moving picture on a picture bypicture basis, the moving picture decoding method comprising: a decodingstep for decoding a signal indicating a picture which is an anchor forrestricting a candidate for a reference picture which is to be referredto for decoding a picture to be decoded; and a determining step fordetermining, as a reference picture, only the picture which is an anchorand intra decoded or one of pictures which have been decoded followingsaid picture, when decoding a picture following said picture.

[0033] In addition, the moving picture decoding method according to thepresent invention is a method for decoding a moving picture on a pictureby picture basis, the moving picture decoding method comprising: adecoding step for decoding a signal indicating a picture which is ananchor for restricting a candidate for a reference picture which isreferred to for decoding a picture to be decoded; a step for deleting apicture which has been decoded prior to the picture which is an anchorand intra decoded; and a determining step for determining, as areference picture, only the picture which is an anchor and intra decodedor one of pictures which have been decoded following said picture, whendecoding a picture following said picture.

[0034] The moving picture coding apparatus according to the presentinvention is a moving picture coding apparatus for coding a movingpicture on a picture by picture basis, the moving picture codingapparatus comprising: a determination unit operable to determine, as areference picture, one of an intra coded picture and pictures which havebeen coded following the intra coded picture, from among a plurality ofpictures stored in a memory; a coding unit operable to read out thedetermined reference picture from the memory, calculate a differentialpicture signal that is a difference between the read-out referencepicture and a picture to be coded, and code the calculated differentialpicture signal; and a storage unit operable to decode the codeddifferential picture signal, add the decoded differential picture signalwith a picture signal of the reference picture, and store a resultingpicture in the memory.

[0035] Also, the moving picture decoding apparatus according to thepresent invention is a moving picture decoding apparatus for decoding amoving picture on a picture by picture basis, the moving picturedecoding apparatus comprising: a decoding unit operable to decode aninputted coded picture signal; a determination unit operable todetermine, as a reference picture, one of an intra decoded picture andpictures which have been decoded following the intra decoded picture,from among a plurality of pictures stored in a memory; and a storageunit operable to read out the determined reference picture from thememory, add a picture signal of the read-out reference picture with adecoded differential picture signal of a picture to be decoded, andoutput a resulting picture to the outside and store the resultingpicture in the memory.

[0036] Further, the present invention can be realized as a program forcausing a computer to execute the steps in the above-mentioned movingpicture coding method and the moving picture decoding method, or asstream data which is coded by the above-mentioned moving picture codingmethod, so as to be distributed via a recording medium such as a CD-ROMor a transmission medium such as a communication network.

[0037] For example, it is assumed that the pictures are coded in theorder of Picture J_(i), Picture J₂, Picture J₃, Picture J₄ and PictureJ₅, Picture J₂ is an intra coded picture and Picture J₁, Picture J₃,Picture J₄ and Picture J₅ are inter coded pictures. Specifically, theirorder in a bit stream is Picture J₁, Picture J₂, Picture J₃, Picture J₄and Picture J₅.

[0038] In this case, in the moving picture coding method and the movingpicture decoding method according to the present invention, thefollowing pictures can be selected as reference pictures for interpicture coding and decoding: Picture J₂, Picture J₃ and Picture J₄ canbe referred to for coding Picture J₅, for example; and Picture J₂ andPicture J₃ can only be referred to for coding Picture J₄ because PictureJ₁ preceding the intra coded Picture J₂ is disabled for reference.

BRIEF DESCRIPTION OF DRAWINGS

[0039]FIG. 1 is an illustration of a concept of a conventional movingpicture coding method and moving picture decoding method.

[0040]FIG. 2 is a block diagram showing a structure of a conventionalmoving picture coding apparatus.

[0041]FIG. 3 is a block diagram showing a structure of a conventionalmoving picture decoding apparatus.

[0042]FIG. 4 is a block diagram showing a structure of a moving picturecoding apparatus of a first embodiment according to the presentinvention.

[0043]FIG. 5 is a flowchart showing operation of a motion estimationunit in the above embodiment.

[0044]FIG. 6 is an illustration showing pictures which can be selectedas reference pictures for coding pictures in the above embodiment.

[0045]FIG. 7 is a block diagram showing another example of the movingpicture coding apparatus of the first embodiment according to thepresent invention.

[0046]FIG. 8 is a block diagram showing a structure of a moving picturedecoding apparatus of the first embodiment according to the presentinvention.

[0047]FIG. 9 is a flowchart showing operation of a motion reconstructionunit in the above embodiment.

[0048]FIG. 10 is an illustration explaining selection of referencepictures used for decoding by methods 1˜3 in the above embodiment, and Ais an illustration of reference pictures used for decoding, and B is anillustration showing picture location relation.

[0049]FIG. 11 is a block diagram showing a structure of a moving picturedecoding apparatus of a second embodiment according to the presentinvention.

[0050]FIG. 12 is a flowchart showing operation of a motionreconstruction unit in the above embodiment.

[0051]FIG. 13 is an illustration of a recording medium for storing aprogram for realizing the moving picture coding method and the movingpicture decoding method of the above-mentioned first embodiment and thesecond embodiment by a computer system. A is an illustration showing anexample of a physical format of a flexible disk that is the recordingmedium main body, B is an illustration showing the front view of theappearance of the flexible disk, the cross-sectional view thereof andthe flexible disk, and C is an illustration showing the structure forrecording and reproducing the above program on the flexible disk FD.

[0052]FIG. 14 is a block diagram showing an overall configuration of acontent providing system which realizes content distribution services.

[0053]FIG. 15 is a diagram showing an example of a mobile phone.

[0054]FIG. 16 is a block diagram showing an internal structure of themobile phone.

[0055]FIG. 17 is a block diagram showing an overall configuration of adigital broadcasting system.

BEST MODE FOR CARRYING OUT THE INVENTION

[0056] The embodiments of the present invention will be explained belowwith reference to FIG. 4˜FIG. 17.

[0057] (First Embodiment)

[0058]FIG. 4 is a block diagram showing a structure of a moving picturecoding apparatus of a first embodiment according to the presentinvention. Note that the same signs are assigned to the devices whichoperate in the same manner as the units of the conventional movingpicture coding apparatus 4 as shown in FIG. 2.

[0059] A moving picture coding apparatus 1 is an apparatus thatcompresses and codes an inputted picture signal Vin, transforms it intoa bit stream by variable length coding or the like and outputs theresulting coded picture signal Str, and is comprised of a motionestimation unit 101, the selection unit 402, the picture signalsubtraction unit 403, the coding unit 404, the decoding unit 405, theaddition unit 406, the selection unit 407, the memories 408˜410 and acounter unit 102.

[0060] When receiving the instruction of intra picture coding by anintra picture coding instruction signal Reset inputted from outside, thecounter unit 102 start counting the number of pictures following anintra coded picture, and notifies the motion estimation unit 101 of thecounting result as the number of potential reference pictures (pictureswhich can be referred to) Num.

[0061] The motion estimation unit 101 compares a picture signal Vin withonly reference pictures which have been previously coded and decodedintra coded picture and the following pictures from among the referencepicture Ref1, the reference picture Ref2 and the reference picture Ref3which are stored in the memories 408 410, respectively, based on thenumber of potential reference pictures, and determines motioninformation MV indicating the reference picture Ref whose inter picturedifferential value (error energy) is smaller and the pixel location formaking the inter picture differential value smaller. Note that thereference picture Ref and the pixel location for making the error energysmallest is usually determined, but the motion information MV may bedetermined for not just for making the error energy smallest, but formaking the error energy smaller and making the compression ratio larger.A reference picture instruction signal RefForm is outputted to theselection unit 402 in order to indicate the reference picture Ref.

[0062] The selection unit 402 outputs the reference picture Ref selectedfrom among the reference picture Ref1, the reference picture Ref2 andthe reference picture Ref3 which are stored in the memories 408˜410,based on a reference picture instruction signal RefForm that is aswitching instruction signal.

[0063] The subtraction unit 403 calculates a differential picture signalDif between the picture signal Vin and the reference picture Ref. Thecoding unit 404 codes the differential picture signal Dif and the motioninformation MV that is the information for identifying a referencepicture, and outputs a coded picture signal Str and coded data Coded.Here, the coded data Coded is the data necessary for reconstructing apicture (data obtained by coding the motion information MV and thedifferential picture signal Dif), and the coded picture signal Str is abit stream obtained by further transforming the coded data Coded byvariable length coding or the like.

[0064] The decoding unit 405 decodes coded data Coded to obtain areconstructed differential picture signal RecDif. The addition unit 406adds the reference picture Ref and the reconstructed differentialpicture signal RecDif. The selection unit 407 outputs the inputteddecoded picture signal Recon to any of the memories 408˜410 as a decodedpicture signal Rec1, a decoded picture signal Rec2 or a decoded picturesignal Rec3, so that the decoded picture signal Recon can be referred toas a reference picture for coding the following pictures. In the presentembodiment, the selection unit 407 switches the memories so that thedecoded picture signal stored in the memory at the earliest time isoverwritten by a new decoded picture signal Recon.

[0065] Next, the operation of the moving picture coding apparatusstructured as above will be explained.

[0066]FIG. 5 is a flowchart showing the operation of the motionestimation unit 101.

[0067] The picture signal Vin is inputted to the picture signalsubtraction unit 403 and the motion estimation unit 101.

[0068] Upon receipt of the picture signal Vin, the motion estimationunit 101 reads out the reference picture Ref1 stored in the memory 408(Step S1). Next, the motion estimation unit 101 judges whether or notthis reference picture Ref1 is an intra coded picture or one of thefollowing pictures. Specifically, it judges using an equation (A), forexample, based on the picture number specific to each picture and thenumber of potential reference pictures which is notified from thecounter unit 102 (Step S2). Here, the picture number is an ID numberassigned to a coded picture and has the following feature.

[0069] Specifically, the picture number of a picture signal Vin islarger by 1 than the picture number of a picture which has been storedmost recently among the pictures which have been stored in the memories408˜410 and are to be reference pictures.

[0070] Picture number of a reference picture

≧Picture number of Picture signal Vin−Number of potential referencepictures Num  (A)

[0071] As a result, when the above equation (A) is satisfied, the motionestimation unit 101 calculates the differential value between thepicture signal Vin and the reference picture Ref1 because the referencepicture Ref1 is an intra coded picture or the following picture. (StepS3), while it does not calculate the differential value when theequation (A) is not satisfied.

[0072] Next, the motion estimation unit 101 performs the processing forthe above-mentioned reference picture Ref1 respectively for thereference picture Ref2 stored in the memory 409 and the referencepicture Ref3 stored in the memory 410 in the same manner (Step S1˜StepS3).

[0073] Next, the motion estimation unit 101 determines a referencepicture whose inter picture differential value is smallest from amongthe reference pictures whose differential values have been calculated asabove (Step S4). Then, the motion estimation unit 101 outputs theinformation for identifying the determined reference picture as motioninformation MV, and, at the same time, outputs a reference pictureinstruction signal RefForm that is a switching instruction signal sothat the selection unit 402 can select the determined reference pictureto output it as a reference picture Ref (Step S5).

[0074] Note that a reference picture whose inter picture differentialvalue is smallest may be determined in Step S4 after the referencepictures whose differential values have been calculated in Step S3 so asto repeat Step S1, Step S2, Step S3 and Step S4 for the memories 408˜410respectively.

[0075] On the other hand, upon receipt of the picture signal Vin, thesubtraction unit 403 calculates a difference between this picture signalVin and the reference picture Ref selected by the selection unit 402,and outputs a differential picture signal Dif to the coding unit 404.Next, the coding unit 404 codes the differential picture signal Dif andthe motion information MV outputted from the motion estimation unit 101to output a coded picture signal Str and coded data Coded.

[0076] The decoding unit 405 decodes the coded data Coded and outputsthe reconstructed differential picture signal RecDif to the additionunit 406. The addition unit 406 adds the reconstructed differentialpicture signal RecDif and the reference picture Ref selected by theselection unit 402, and outputs the decoded picture signal Recon to theselection unit 407. The selection unit 407 outputs the decoded picturesignal Recon to any of the memories 408˜410 as a decoded picture signalRec1, a decoded picture signal Rec2 or a decoded picture signal Rec3 soas to enable the decoded picture signal Recon to be referred to forcoding the following pictures.

[0077]FIG. 6 is an illustration showing pictures which can be selectedas reference pictures for coding pictures. As is the case of FIG. 1,Picture J₂ is an intra coded picture, and Picture J₁, Picture J₃,Picture J₄ and Picture J₅ are inter coded pictures. The coding order ina bit stream is Picture J₁, Picture J₂, Picture J₃, Picture J₄ andPicture J₅.

[0078] As mentioned above, when inter picture coding is performed underthe operation of the moving picture coding apparatus 1, for example,when Picture J₅ is coded as shown in FIG. 6, Picture J₂, Picture J₃ andPicture J₄ can be selected as reference pictures. Also, when Picture J4is coded, only Picture J₂ and Picture J₃ can be referred to becausePicture J₁ preceding the intra coded Picture J₂ is disabled for coding.

[0079] As described above, since pictures are coded using only an intracoded picture and pictures which have been coded following the intracoded picture as reference pictures, reproduction can be started fromthe intra coded picture at some midpoint of a coded picture signal.Also, even if an error occurs in a stream, a coded picture signal Str,that is, a stream which can be reproduced without error following theintra coded picture, can be generated.

[0080] In order to start reproduction at some midpoint of a picturesignal recorded on a DVD or a hard disk, a mechanism for the midpointreproduction is required, so Reset is instructed at the picture fromwhich reproduction is to be started. This Reset instruction may bedetermined and made by an operator when coding pictures, or at everypredetermined picture cycle or time lapse.

[0081] Note that each picture has the information on whether the pictureis intra coded or inter coded as auxiliary information. So, a signal,indicating that the intra coded picture which has been explained in theabove first embodiment is specified by an intra picture codinginstruction signal Reset, may also be coded (FIG. 7). Specifically, ifthis is the intra coded picture specified by this intra picture codinginstruction signal Reset, it may be designed so that pictures stored ina memory prior to this specified picture are not referred to.

[0082] Or, it means here that pictures stored in a memory prior to anintra coded picture are not referred to, with the intra coded pictureset as the anchor picture, but the pictures stored in the memory priorto the intra coded picture may be deleted from the memory, with theintra coded picture set as the anchor picture.

[0083] In addition, an intra picture coding instruction signal Reset,which indicates that pictures stored in a memory prior to an intra codedpicture are not referred to, is coded here, but in order to delete thepictures stored prior to the intra coded picture from the memory, acontrol command other than this intra picture coding instruction signalReset may be coded.

[0084]FIG. 8 is a block diagram showing the structure of a movingpicture decoding apparatus of the first embodiment according to thepresent invention. Note that the same signs are assigned to the unitswhich operate in the same manner as the units in the conventional movingpicture decoding apparatus 5 as shown in FIG. 3.

[0085] The moving picture decoding apparatus 2 is an apparatus fordecoding a coded picture signal Str which is coded by the moving picturecoding apparatus as shown in the above first embodiment, and iscomprised of a decoding unit 201, a motion reconstruction unit 202, theselection unit 503, the addition unit 504, the selection unit 505, thememories 506˜508, and a counter unit 203.

[0086] The decoding unit 201 decodes the inputted coded picture signalStr, and outputs a reconstructed differential picture signal RecDif,motion information MV and an intra picture coding instruction signalReset. Upon receipt of the instruction of intra picture coding by theintra picture coding instruction signal Reset, the counter unit 203starts counting the number of pictures following an intra coded picture,and notifies the motion reconstruction unit 202 of the result as thenumber of potential reference pictures Num.

[0087] The motion reconstruction unit 202 decodes the motion informationMV, judges based on the number of potential reference pictures whether areference picture indicated by this motion information MV is a picturewhich can be referred to or not, and determines the reference picture.

[0088] The selection unit 503 selects one picture from among thereference picture Ref1, the reference picture Ref2 and the referencepicture Ref3 stored in the memories 506˜508 based on a reference pictureinstruction signal RefForm that is a switching instruction signal, andoutputs the picture as a reference picture Ref. The addition unit 504adds the reconstructed differential picture signal RecDif and thereference picture Ref to output a decoded picture signal Vout1 (whichcorresponds to a decoded picture signal Recon in FIG. 4).

[0089] The selection unit 505 outputs the inputted decoded picturesignal Vout1 to any of the memories 506˜508 as the decoded picturesignal Rec1, the decoded picture signal Rec2 or the decoded picturesignal Rec3 in order to enable the decoded picture signal Vout1 to bereferred to as a reference picture when decoding the following pictures.In the present embodiment, the selection unit 505 switches the memoriesso that a picture which has been stored in any of these memories at theearliest time is overwritten by a new decoded picture signal Recon.

[0090] Next, the operation of the moving picture decoding apparatusstructured as mentioned above will be explained.

[0091]FIG. 9 is a flowchart showing the operation of the motionreconstruction unit 202.

[0092] The coded picture signal Str is inputted to the decoding unit201. The decoding unit 201 decodes the inputted coded picture signal Strand outputs a reconstructed differential picture signal RecDif andmotion information MV. Also, when the inputted coded picture signal Stris an intra coded picture, the decoding unit 201 outputs an intrapicture coding instruction signal Reset.

[0093] Upon receipt of the intra picture coding instruction signal Resetfrom the decoding unit 201, the counter unit 203 starts calculating thenumber of pictures following the intra coded picture, and notifies themotion reconstruction unit 202 of the result as the number of potentialreference pictures Num.

[0094] Upon receipt of the motion information MV and the number ofpotential reference pictures Num, the motion reconstruction unit 202identifies the reference picture which has been referred to for codingbased on the motion information MV that is the information foridentifying a reference picture (Step S11). Next, the motionreconstruction unit 202 judges whether or not this reference picture isan intra decoded picture or one of the following pictures. Specifically,it judges using an equation (B), for example, based on the picturenumber specific to each picture and the number of potential referencepictures which is notified from the counter unit 203 (Step S12).

[0095] Picture number of a reference picture

≧Picture number of coded picture signal Str−Number of potentialreference pictures Num  (B)

[0096] As a result, since the reference picture is an intra decodedpicture or one of the following pictures when the above equation (B) issatisfied, the motion reconstruction unit 202 selects the referencepicture identified based on the motion information MV as a referencepicture used for decoding (Step S13).

[0097] On the other hand, when the equation (B) is not be satisfied, thereference picture is an intra decoded picture. Basically, pictures musthave been coded with reference only to an intra coded picture or thefollowing pictures. Therefore, the reference picture necessary fordecoding which has been identified based on the motion information MVshould be an intra decoded picture or one of pictures which have beendecoded following the intra decoded picture and obtained based on thenumber of potential reference pictures Num. However, there is apossibility that a picture which has been decoded prior to the intradecoded picture is referred to, that is, the above equation (B) is notbe satisfied, due to a transmission error or the like. So, the motionreconstruction unit 202 selects a reference picture used for decodingaccording to any of the following methods 1˜3 for predetermining thereference picture in the case when the equation (B) is not be satisfied(Step S14).

[0098]FIG. 10 is an illustration explaining selection of a referencepicture used for decoding by methods 1˜3, and A is an illustration of areference picture used for decoding, and B is an illustration showingpicture location relation. This example shows the decoding of PictureP₄, and Picture P₂ is an intra decoded picture and the other Picture P₁,Picture P₃ and Picture P₄ are inter decoded pictures. In both thedisplay order and decoding start (location in a stream) order, thesepictures are ordered, from earliest to latest, in the order of PictureP₁, Picture P₂, Picture P₃ and Picture P₄.

[0099] (Method 1)

[0100] A reference picture (Picture P₁) identified based on the motioninformation MV is selected as a reference picture to be used fordecoding as it is. In this case, even if a picture preceding an intracoded picture has been referred to for coding due to error, the picturecan be properly decoded if only Picture P₁ has been decoded properly.

[0101] (Method 2)

[0102] An intra decoded picture (Picture P₂) is selected as a referencepicture to be used for decoding. In this case, since the intra decodedpicture is a picture which has been decoded earliest as a referencepicture among potential reference pictures, the intra decoded picturehas the strongest correlation with a picture preceding itself among thepotential reference pictures, and thus there is a high possibility thatthe picture quality is not degraded even if the reference picture usedfor coding is a picture which has been decoded prior to the intradecoded picture.

[0103] (Method 3)

[0104] A picture which has been decoded most recently (Picture P₃) isselected as a reference picture to be used for decoding. Generally,picture signals have stronger correlation with each other if the timeinterval between them is shorter, and, as a result, there is a very highpossibility that the picture which has been decoded most recently is tobe a reference picture. Therefore, the most recently decoded picturewith the strongest correlation is most likely to be a proper referencepicture if a wrong reference picture is identified based on the motioninformation MV, and thus there is a high possibility that the picturequality is not degraded.

[0105] Note that any one of these methods 1˜3 may be used, or acombination of two or more methods may be used. As an example of such acombination, when in the method 1, the reference picture (Picture P₁)identified based on the motion information MV cannot be selected as areference picture to be used for decoding as it is, default processingmay be performed for selecting the picture decoded most recently, asshown in the method 3, as a reference picture to be used for decoding,for example.

[0106] Next, the motion reconstruction unit 202 reads out in sequencethe reference picture Ref1, reference picture Ref2 and reference pictureRef3 stored in the memories 506˜508, and judges whether they are thereference pictures selected as mentioned above or not (Step S15 in FIG.9). In other words, it identifies the memory storing the selectedreference picture among the memories 506˜508. Then, the motionreconstruction unit 202 outputs a reference picture instruction signalRefForm that is a switching instruction signal so that the selectionunit 503 can select for outputting the identified reference pictureRef1, reference picture Ref2 or reference picture Ref3 as a referencepicture Ref (Step S16 in FIG. 9).

[0107] The addition unit 504 adds the reference picture Ref and thereconstructed differential picture signal RecDif outputted from thedecoding unit 201, and outputs a decoded picture signal Vout1 (whichcorresponds to a decoded picture signal Recon in FIG. 4) to theselection unit 505. The selection unit 505 outputs the inputted decodedpicture signal Vout1 to any of the memories 506 508 as a decoded picturesignal Rec1, decoded picture signal Rec2 or decoded picture signal Rec3,so as to enable the decoded picture signal Vout1 to be referred to as areference picture when decoding the following pictures.

[0108] As described above, a coded picture signal Str which has beencoded with reference only to an intra decoded picture or picturesfollowing the intra decoded picture as reference pictures can beproperly decoded to obtain a decoded picture signal Vout1, and thusreproduction can be started from the intra coded picture at somemidpoint of a coded picture signal. Also, even if an error occurs in astream, pictures following the intra coded picture can be reproducedwithout error.

[0109] Note that in the present embodiment, the methods 1˜3 arepredetermined for selecting a reference picture to be used by the motionreconstruction unit 202 for decoding, but the present invention is notlimited to these methods. These three methods or two of them may bedynamically switched according to the situation, in such a way that, forexample, the method 3 is used when the picture number identified basedon motion information MV is quite different from the potential referencepicture numbers, the method 1 is used when the picture identified basedon the motion information MV is stored in a memory and can be referredto, and the method 2 is used in other cases.

[0110] Also, each picture has the information on whether the picture isto be intra decoded or inter decoded as auxiliary information. So, thecoded picture signal Str, including a signal indicating that the intracoded picture which has been explained in the above first embodiment isspecified by an intra picture coding instruction signal Reset, may bereceived, and that signal indicating the picture specified by the intrapicture coding instruction signal Reset may be decoded. Specifically, ifa picture is an intra coded picture specified by this intra picturecoding instruction signal Reset, it may be designed so that picturesstored in a memory prior to this specified picture are not referred to.

[0111] Or, it means here that pictures stored in a memory prior to anintra coded picture are not referred to, with the intra decoded pictureset as the anchor picture, but pictures stored in a memory prior to theintra decoded picture may be deleted from the memory, with the intradecoded picture set as the anchor picture.

[0112] In addition, in order to delete the pictures stored prior to theintra coded picture from the memory, it may be designed so as to receivea coded picture signal Str including a control command other than thisintra picture coding instruction signal Reset, decode this controlcommand, judge whether or not this control command is received alongwith the signal indicating the picture specified by the intra picturecoding instruction signal Reset, and perform error control.

[0113] (Second Embodiment)

[0114] Next, a moving picture decoding apparatus of a second embodimentof the present invention will be explained.

[0115]FIG. 11 is a block diagram showing the structure of the movingpicture decoding apparatus of the second embodiment according to thepresent invention. Note that the same signs are assigned to the unitswhich operate in the same manner as those of the moving picture decodingapparatus 2 as shown in FIG. 8 A moving picture decoding apparatus 3includes a selection unit 301 in addition to the units of the movingpicture decoding apparatus 2 as shown in FIG. 8, and it is differentfrom the first embodiment in its operation in the case where a referencepicture identified by motion information MV is a picture which has beendecoded prior to an intra decoded picture due to transmission error orthe like.

[0116] The selection unit 301 outputs either a decoded picture signalRecon outputted from the addition unit 504 or a reference picture Refoutputted from the selection unit 503 as a decoded picture signal Vout2,based on an error notice signal Err notified from a motionreconstruction unit 302.

[0117] Next, the operation of the moving picture decoding apparatusstructured as mentioned above will be explained, but the explanation ofthe portions same as the above first embodiment will be omitted.

[0118]FIG. 12 is a flowchart showing the operation of the motionreconstruction unit 302.

[0119] The operation in Steps S21˜S22 as shown in FIG. 12 is same as theoperation in Steps S11˜S12 as shown in FIG. 9. Next, as a result ofjudging whether the identified reference picture is an intra decodedpicture or one of the following pictures or not, if the above equation(B) is not satisfied, that is, the reference picture is a picture whichhas been decoded prior to the intra decoded picture, the motionreconstruction unit 302 sends an error notice signal Err to theselection unit 301 to notify it of an error. The motion reconstructionunit 302 also selects the picture decoded most recently as a referencepicture and a decoded picture signal (Step S23). In other words, whenreceiving the error notice signal Err, the selection unit 301 outputs areference picture Ref as a decoded picture signal Vout2, so selectingthe picture decoded most recently as a reference picture means selectingthe picture decoded most recently as a decoded picture signal.

[0120] On the other hand, when the above equation (B) is satisfied as aresult of the above judgment, the reference picture is an intra decodedpicture or one of the following pictures, so the motion reconstructionunit 302 selects, in the same manner as the above first embodiment, areference picture identified based on the motion information MV as areference picture used for decoding (Step S24).

[0121] The operation in the next Steps S25˜S26 as shown in FIG. 12 issame as the operation in the Steps S15˜S16 as shown in FIG. 9.

[0122] Next, the addition unit 504 adds the reference picture Ref andthe reconstructed differential picture signal RecDif outputted from thedecoding unit 201 to output a decoded picture signal Recon. When theerror notice signal Err is not notified from the motion reconstructionunit 302, the selection unit 301 outputs the decoded picture signalRecon outputted from the addition unit 504 as a decoded picture signalVout2, and when the error notice signal Err is notified, it outputs thereference picture Ref outputted from the selection unit 503 as a decodedpicture signal vout2. In sum, when the error notice signal Err isnotified, it outputs the picture decoded most recently as it is as adecoded picture signal Vout2.

[0123] The selection unit 505 outputs the inputted decoded picturesignal Vout2 to any of the memories 506˜508 as a decoded picture signalRec1, a decoded picture signal Rec2 or a decoded picture signal Rec3 soas to enable the decoded picture signal Vout2 to be referred to as areference picture when decoding the following pictures.

[0124] As described above, even if a picture preceding an intra decodedpicture is referred to due to a transmission error or the like, areference picture Ref that is the picture decoded most recently with thestrongest pixel correlation is outputted as a decoded picture signalVout2, so the influence on picture degradation due to such an error canbe minimized.

[0125] Note that in each of the above embodiments, the maximum of 3coded pictures can be referred to because the number of memories is 3,but the present invention is not limited to that number, but coding anddecoding can be performed with reference to more coded pictures with theincrease of the number of memories.

[0126] Also, in each of the above embodiments, motion compensation unitsnot shown in the figures are respectively provided on the output sidesof the selection unit 402 and the selection unit 503 of the movingpicture coding apparatus 1 and the moving picture decoding apparatuses 2and 3, for performing motion compensation, that is, compensation ofpixel motion between pictures.

[0127] In addition, in each of the above embodiments, reference picturesare limited for inter picture coding, so that any pictures which havebeen coded prior to an intra coded picture, if any, are not referred toas reference pictures, but the present invention is not limited to that.For example, it is also possible not to limit reference pictures so thatany pictures decoded prior to an intra coded picture are not referred toas reference pictures. In other words, it is possible to provide anothertype of an intra coded picture so that the pictures coded prior to theintra coded picture can be referred to as reference pictures, as is thecase of a traditional method, and use both types of intra coded picturesaccording to need. In this case, the information for distinguishing twotypes of intra coded pictures can be stored in header information or thelike in a coded picture signal Str, for example.

[0128] Further, in each of the above embodiments, the counter unit 102or 203 may count in picture display order, not in picture coding ordecoding order.

[0129] (Third Embodiment)

[0130] In addition, if a program for realizing the structure of thepicture coding method or the picture decoding method as shown in each ofthe above-mentioned embodiments is recorded on a storage medium such asa flexible disk, it becomes possible to perform the processing as shownin each of the above embodiments easily in an independent computersystem.

[0131]FIG. 13 is an illustration showing the implementation by acomputer system using a flexible disk which stores the picture codingmethod or the picture decoding method in the above-mentioned first andsecond embodiments.

[0132]FIG. 13B shows the front view and the cross-sectional view of theappearance of a flexible disk, and the flexible disk, and FIG. 13A showsan example of a physical format of a flexible disk as a recording mediumitself. A flexible disk FD is contained in a case F, a plurality oftracks Tr are formed concentrically on the surface of the disk in theradius direction from the periphery, and each track is divided into 16sectors Se in the angular direction. Therefore, as for the flexible diskstoring the above-mentioned program, the picture coding method and thepicture decoding method as the above program are recorded in an areaallocated for it on the flexible disk FD.

[0133]FIG. 13C shows the structure for recording and reproducing theprogram on and from the flexible disk FD. For recording the program onthe flexible disk FD, the computer system Cs writes the picture codingmethod or the picture decoding method as the program on the flexibledisk FD via a flexible disk drive FDD. For constructing the abovepicture coding method and the picture decoding method in the computersystem by the program recorded on the flexible disk, the program is readout from the flexible disk FD via the flexible disk drive FDD andtransferred to the computer system.

[0134] Note that the above explanation is made on the assumption that arecording medium is a flexible disk, but the same processing can also beperformed using an optical disk. In addition, the recording medium isnot limited to these, but any other mediums such as an IC card and a ROMcassette can be used in the same manner if a program can be recorded onthem.

[0135] Further, the applications of the moving picture coding method andthe moving picture decoding methods as shown in the above embodimentsand a system using them will be explained here.

[0136]FIG. 14 is a block diagram showing the overall configuration of acontent providing system ex100 for realizing content distributionservice. The area for providing communication service is divided intocells of desired size, and base stations ex107˜ex110 which are fixedwireless stations are placed in respective cells.

[0137] This content providing system ex100 is connected to variousdevices such as a computer ex111, a PDA (personal digital assistant)ex112, a camera ex113, a mobile phone ex114 and a camera-equipped mobilephone ex115 via the Internet ex101, an Internet service provider ex102,a telephone network ex104 and base stations ex107˜ex110, for example.

[0138] However, the content providing system ex100 is not limited to thecombination as shown in FIG. 14, and may be connected to a combinationof any of them. Also, each device may be connected directly to thetelephone network ex104, not through the base stations ex107˜ex110 whichare the fixed wireless stations.

[0139] The camera ex113 is a device such as a digital video cameracapable of shooting moving pictures. The mobile phone may be any of amobile phone of a PDC (Personal Digital Communications) system, a CDMA(Code Division Multiple Access) system, a W-CDMA (Wideband-Code DivisionMultiple Access) system or a GSM (Global System for MobileCommunications) system, a PHS (Personal Handyphone System) and the like.

[0140] Also, a streaming server ex103 is connected to the camera ex113via the base station ex109 and the telephone network ex104, whichenables live distribution or the like using the camera ex113 based onthe coded data transmitted from the user. Either the camera ex113 or theserver for transmitting the data may code the data shot by the camera.Also, the moving picture data shot by a camera ex116 may be transmittedto the streaming server ex103 via the computer ex111. The camera ex116is a device such as a digital camera capable of shooting still andmoving pictures. In this case, either the camera ex116 or the computerex111 may code the moving picture data. An LSI ex117 included in thecomputer ex111 or the camera ex116 performs coding processing. Note thatsoftware for coding and decoding pictures may be integrated into anytype of a storage medium (such as a CD-ROM, a flexible disk and a harddisk) that is a recording medium which can be read by the computer ex111or the like. Furthermore, the camera-equipped mobile phone ex115 maytransmit the moving picture data. This moving picture data is the datacoded by the LSI included in the mobile phone ex115.

[0141] In this content providing system ex100, contents (such as a musiclive video) shot by users using the camera ex113, the camera ex116 orthe like are coded in the same manner as the above embodiments andtransmitted to the streaming server ex103, while the streaming serverex103 makes stream distribution of the above content data to the clientsat their request. The clients include the computer ex111, the PDA ex112,the camera ex113, the mobile phone ex114 and so on capable of decodingthe above-mentioned coded data. The content providing system ex100 is asystem in which, the clients can thus receive and reproduce the codeddata, and further can receive, decode and reproduce the data in realtime so as to realize personal broadcasting.

[0142] When each device in this system performs coding or decoding, themoving picture coding apparatus or the moving picture decoding apparatusas shown in each of the above-mentioned embodiments may be used.

[0143] A mobile phone will be explained as an example thereof.

[0144]FIG. 15 is a diagram showing a mobile phone ex115 which uses themoving picture coding method and the moving picture decoding method asexplained in the above embodiments. The mobile phone ex115 has anantenna ex201 for sending and receiving radio waves between the basestation ex110, a camera unit ex203 such as a CCD camera capable ofshooting video and still pictures, a display unit ex202 such as a liquidcrystal display for displaying the data obtained by decoding video shotby the camera unit ex203, video received by the antenna ex201, or thelike, a main body including a set of operation key s ex204, a voiceoutput unit ex208 such as a speaker for outputting voices, a voice inputunit ex205 such as a microphone for inputting voices, a storage mediumex207 for storing coded or decoded data, such as data of moving or stillpictures shot by the camera, and data of text, moving pictures or stillpictures of received e-mails, and a slot unit ex206 for attaching thestorage medium ex207 into the mobile phone ex115. The storage mediumex207 includes a flash memory element, a kind of EEPROM (ElectricallyErasable and Programmable Read Only Memory) that is an electricallyerasable and rewritable nonvolatile memory, in a plastic case such as anSD ca rd.

[0145] Further, the mobile phone ex115 will be explained with referenceto FIG. 16. In the mobile phone ex115, a main control unit ex311 foroverall controlling each unit of the display unit ex202 and the mainbody including operation keys ex204 is connected to a power supplycircuit unit ex310, an operation input control unit ex304, a picturecoding unit ex312, a camera interface unit ex303, an LCD (Liquid CrystalDisplay) control unit ex302, a picture decoding unit ex309, amultiplex/demultiplex unit ex308, a record/reproduce unit ex307, a modemcircuit unit ex306 and a voice processing unit ex305 to each other via asynchronous bus ex313.

[0146] When a call-end key or a power key is turned ON by a user'soperation, the power supply circuit unit ex310 supplies respective unitswith power from a battery pack so as to activate the camera-equippeddigital mobile phone ex115 for a ready state.

[0147] In the mobile phone ex115, under the control of the main controlunit ex311 including a CPU, ROM, RAM and the like, the voice processingunit ex305 converts the voice signals received by the voice input unitex205 in conversation mode into digital voice data, the modem circuitunit ex306 performs spread spectrum processing of the digital voicedata, and the send/receive circuit unit ex301 performs digital-to-analogconversion and frequency transformation of the data, so as to transmitthe result via the antenna ex201. Also, in the mobile phone ex115, thedata received by the antenna ex201 in conversation mode is amplified andperformed of frequency transformation and analog-to-digital conversion,the modem circuit unit ex306 performs inverse spread spectrum processingof the data, and the voice processing unit ex305 converts it into analogvoice data, so as to output the result via the voice output unit ex208.

[0148] Furthermore, when transmitting an e-mail in data communicationmode, the text data of the e-mail inputted by operating the operationkeys ex204 on the main body is sent out to the main control unit ex311via the operation input control unit ex304. In the main control unitex311, after the modem circuit unit ex306 performs spread spectrumprocessing of the text data and the send/receive circuit unit ex301performs digital-to-analog conversion and frequency transformation ofit, the result is transmitted to the base station ex110 via the antennaex201.

[0149] When picture data is transmitted in data communication mode, thepicture data shot by the camera unit ex203 is provided to the picturecoding unit ex312 via the camera interface unit ex303. When the picturedata is not transmitted, the picture data shot by the camera unit ex203can also be displayed directly on the display unit 202 via the camerainterface unit ex303 and the LCD control unit ex302.

[0150] The picture coding unit ex312, including the picture codingapparatus explained in the present invention, compress and codes thepicture data provided from the camera unit ex203 by the coding methodused for the picture coding apparatus as shown in the above-mentionedembodiments so as to transform it into coded picture data, and sends itout to the multiplex/demultiplex unit ex308. At this time, the mobilephone ex115 sends out the voices received by the voice input unit ex205during picture pickup by the camera unit ex203 to themultiplex/demultiplex unit ex308 as digital voice data via the voiceprocessing unit ex305.

[0151] The multiplex/demultiplex unit ex308 multiplexes the codedpicture data provided from the picture coding unit ex312 and the voicedata provided from the voice processing unit ex305 by a predeterminedmethod, the modem circuit unit ex306 performs spread spectrum processingof the resulting multiplexed data, and the send/receive circuit unitex301 performs digital-to-analog conversion and frequency transformationof the result for transmitting via the antenna ex201.

[0152] As for receiving data of a moving picture file which is linked toa Website or the like in data communication mode, the modem circuit unitex306 performs inverse spread spectrum processing of the data receivedfrom the base station ex110 via the antenna ex201, and sends out theresulting multiplexed data to the multiplex/demultiplex unit ex308.

[0153] In order to decode the multiplexed data received via the antennaex201, the multiplex/demultiplex unit ex308 demultiplexes themultiplexed data into a coded bit stream of picture data and a coded bitstream of voice data, and provides the coded picture data to the picturedecoding unit ex309 and the voice data to the voice processing unitex305 respectively via the synchronous bus ex313.

[0154] Next, the picture decoding unit ex309, including the picturedecoding apparatus explained in the present invention, decodes the codedbit stream of the picture data by the decoding method paired with thecoding method as shown in the above-mentioned embodiments, so as togenerate reproduced moving picture data, and provides this data to thedisplay unit ex202 via the LCD control unit ex302, and thus movingpicture data included in a moving picture file linked to a Website, forinstance, is displayed. At the same time, the voice processing unitex305 converts the voice data into analog voice data, and provides thisdata to the voice output unit ex208, and thus voice data included in amoving picture file linked to a Website, for instance, is reproduced.

[0155] The present invention is not limited to the above-mentionedsystem. Ground-based or satellite digital broadcasting has been in thenews lately, and at least either the picture coding apparatus or thepicture decoding apparatus in the above-mentioned embodiments can beincorporated into such a digital broadcasting system as shown in FIG.17. More specifically, a coded bit stream of video information istransmitted from a broadcast station ex409 to or communicated with abroadcast satellite ex410 via radio waves. Upon receipt of it, thebroadcast satellite ex410 transmits radio waves for broadcasting, a homeantenna ex406 with a satellite broadcast reception function receives theradio waves, and an apparatus such as a television (receiver) ex401 or aset top box (STB) ex407 decodes the coded bit stream for reproduction.The picture decoding apparatus as shown in the above-mentionedembodiments can be implemented in the reproducing apparatus ex403 forreading a coded bit stream recorded on a storage medium ex402 such as aCD and DVD that is a recording medium and decoding it. In this case, thereproduced video signals are displayed on a monitor ex404. It is alsoconceived to implement the picture decoding apparatus in the set top boxex407 connected to a cable ex405 for a cable television or the antennaex406 for satellite and/or ground-based broadcasting so as to reproducethem on a monitor ex408 of the television ex401. The picture decodingapparatus may be incorporated into the television, not in the set topbox. Or, a car ex412 having an antenna ex411 can receive signals fromthe satellite ex410, the base station ex107 or the like for reproducingmoving pictures on a display apparatus such as a car navigation deviceex413 or the like in the car ex412.

[0156] Furthermore, the picture coding apparatus as shown in theabove-mentioned embodiments can code picture signals for recording themon a recording medium. As a concrete example, there is a recorder ex420such as a DVD recorder for recording picture signals on a DVD disk ex421and a disk recorder for recording them on a hard disk. They can also berecorded on an SD card ex422. If the recorder ex420 includes the picturedecoding apparatus as shown in the above-mentioned embodiments, thepicture signals recorded on the DVD disk ex421 or the SD card ex422 canbe reproduced for display on the monitor ex408.

[0157] Note that as the structure of the car navigation device ex413,the structure without the camera unit ex203, the camera interface unitex303 and the picture coding unit ex312, out of the units as shown inFIG. 16, is conceivable. The same applies to the computer ex111, thetelevision (receiver) ex401 and others.

[0158] In addition, three types of implementations can be conceived fora terminal such as the above-mentioned mobile phone ex114; asending/receiving terminal equipped with both an encoder and a decoder,a sending terminal equipped with an encoder only, and a receivingterminal equipped with a decoder only.

[0159] As described above, it is possible to use the moving picturecoding method or the moving picture decoding method as shown in theabove embodiments in any of above-mentioned devices and systems, andthus the effects explained in the above embodiments can be obtained.

[0160] In addition, the moving picture coding method and the movingpicture decoding method as shown in the above-mentioned embodiments areeffective when stream data recorded on a storage medium such as a DVD,an SD card and a memory is reproduced at some midpoint of the data.

[0161] As is obvious from the above explanation, the moving picturecoding method according to the present invention is a method for codinga moving picture on a picture by picture basis, the moving picturecoding method comprising: a determining step for determining, as areference picture, one of an intra coded picture and pictures which havebeen coded following the intra coded picture, from among a plurality ofpictures stored in a memory; a coding step for reading out thedetermined reference picture from the memory, calculating a differentialpicture signal that is a difference between the read-out referencepicture and a picture to be coded, and coding the calculateddifferential picture signal; and a storing step for decoding the codeddifferential picture signal, adding the decoded differential picturesignal with a picture signal of the reference picture, and storing aresulting picture in the memory.

[0162] Accordingly, since coding is not performed with reference topictures preceding an intra coded picture as reference pictures,reproduction can be started from the intra coded picture at a midpointof a coded picture signal. Also, a coded picture signal, that is astream of pictures which can be reproduced without an error followingthe intra coded picture even if a stream error occurs, can be generated.As mentioned above, the present invention has strong advantages inmidpoint reproduction which is important for storage mediums and errorresilience for transmission which is important for wireless and cabletransmission.

[0163] Also, the moving picture decoding method according to the presentinvention is a method for decoding a moving picture on a picture bypicture basis, the moving picture decoding method comprising: a decodingstep for decoding an inputted coded picture signal; a determining stepfor determining, as a reference picture, one of an intra decoded pictureand pictures which have been decoded following the intra decodedpicture, from among a plurality of pictures stored in a memory; and astoring step for reading out the determined reference picture from thememory, adding a picture signal of the read-out reference picture with adecoded differential picture signal of a picture to be decoded, andoutputting a resulting picture to the outside and storing the resultingpicture in the memory.

[0164] Accordingly, a coded picture signal, which has been coded withreference only to an intra decoded picture or the following pictures asreference pictures, can be properly decoded to obtain a decoded picturesignal, and thus reproduction can be started from the intra codedpicture at some midpoint of the coded picture signal. Also, even if anerror occurs in a stream, the stream can be reproduced without an errorfollowing the intra coded picture.

INDUSTRIAL APPLICABILITY

[0165] As described above, the moving picture coding method and themoving picture decoding method according to the present invention areuseful for starting reproduction from some midpoint of a stream datarecorded on a storage medium such as a DVD, an SD card and a memory usedfor a mobile phone, a DVD apparatus, a personal computer or the like.

1. A moving picture coding method for coding a moving picture on apicture by picture basis, the moving picture coding method comprising: adetermining step for determining, as a reference picture, one of anintra coded picture and pictures which have been coded following theintra coded picture, from among a plurality of pictures stored in amemory; a coding step for reading out the determined reference picturefrom the memory, calculating a differential picture signal that is adifference between the read-out reference picture and a picture to becoded, and coding the calculated differential picture signal; and astoring step for decoding the coded differential picture signal, addingthe decoded differential picture signal with a picture signal of thereference picture, and storing a resulting picture in the memory.
 2. Amoving picture coding method for coding a moving picture on a picture bypicture basis, the moving picture coding method comprising: a specifyingstep for specifying an intra coded picture of a plurality of picturesstored in a memory as a picture which is an anchor for restricting acandidate for a reference picture; and a coding step for coding a signalindicating the picture which is an anchor for restricting a candidatefor a reference picture so that a picture to be coded following thepicture specified in the specifying step refers only to said specifiedpicture or pictures which have been coded following said specifiedpicture.
 3. A moving picture coding method for coding a moving pictureon a picture by picture basis, the moving picture coding methodcomprising: a specifying step for specifying an intra coded picture of aplurality of pictures stored in a memory as a picture which is an anchorfor restricting a candidate for a reference picture; a step for deletinga picture which has been coded prior to the picture specified in thespecifying step; and a coding step for coding a signal indicating thatthe picture which has been coded prior to the picture specified in thespecifying step is deleted so that a picture to be coded following saidspecified picture refers only to said specified picture or pictureswhich have been coded following said specified picture.
 4. The movingpicture coding method according to claim 1, wherein in the determiningstep, the number of pictures which have been coded following the intracoded picture is counted, and the reference picture is determined basedon the counted number.
 5. The moving picture coding method according toclaim 4, wherein picture numbers indicating coding order are assigned tothe pictures stored in the memory, and in the determining step, theplurality of pictures stored in the memory are read out, and whether ornot the respective read-out pictures are the intra coded picture orpictures which have been coded following said intra coded picture isjudged based on whether or not the respective picture numbers for theread-out pictures are a value obtained by subtracting the counted numberfrom a picture number of the picture to be coded or more.
 6. The movingpicture coding method according to claim 5, wherein in the determiningstep, if the read-out pictures are the intra coded picture or thepictures which have been coded following said intra coded picture as aresult of the judgment, differences between the read-out pictures andthe picture to be coded are respectively calculated, and a picture whosecalculated difference is smallest is determined as the referencepicture.
 7. A moving picture decoding method for decoding a movingpicture on a picture by picture basis, the moving picture decodingmethod comprising: a decoding step for decoding an inputted codedpicture signal; a determining step for determining, as a referencepicture, one of an intra decoded picture and pictures which have beendecoded following the intra decoded picture, from among a plurality ofpictures stored in a memory; and a storing step for reading out thedetermined reference picture from the memory, adding a picture signal ofthe read-out reference picture with a decoded differential picturesignal of a picture to be decoded, and outputting a resulting picture tothe outside and storing the resulting picture in the memory.
 8. A movingpicture decoding method for decoding a moving picture on a picture bypicture basis, the moving picture decoding method comprising: a decodingstep for decoding a signal indicating a picture which is an anchor forrestricting a candidate for a reference picture which is to be referredto for decoding a picture to be decoded; and a determining step fordetermining, as a reference picture, only the picture which is an anchorand intra decoded or one of pictures which have been decoded followingsaid picture, when decoding a picture following said picture.
 9. Amoving picture decoding method for decoding a moving picture on apicture by picture basis, the moving picture decoding method comprising:a decoding step for decoding a signal indicating a picture which is ananchor for restricting a candidate for a reference picture which isreferred to for decoding a picture to be decoded; a step for deleting apicture which has been decoded prior to the picture which is an anchorand intra decoded; and a determining step for determining, as areference picture, only the picture which is an anchor and intra decodedor one of pictures which have been decoded following said picture, whendecoding a picture following said picture.
 10. The moving picturedecoding method according to claim 8 or claim 9, wherein the signalindicating the picture which is an anchor is a signal indicating thatsaid picture is intra coded.
 11. The moving picture decoding methodaccording to claim 8 or claim 9, wherein the signal indicating thepicture which is an anchor is another signal than a signal indicatingwhether said picture is intra coded or inter coded.
 12. The movingpicture decoding method according to claim 7, wherein the determiningstep includes: a judging step for determining a picture which should bea reference picture based on decoded information and judging whether ornot the determined picture is the intra decoded picture or one ofpictures which have been decoded following said intra decoded picture; anormal processing step for determining the determined picture as areference picture when the determined picture is the intra decodedpicture or one of the pictures which have been decoded following saidintra decoded picture as a result of the judgment; and an errorprocessing step for determining a predetermined picture as a referencepicture if the determined picture is not the intra decoded picture norone of the pictures which have been decoded following said intra decodedpicture.
 13. The moving picture decoding method according to claim 12,wherein in the judging step, the number of pictures which have beendecoded following the intra decoded picture is counted, and the judgmentis made based on the counted number.
 14. The moving picture decodingmethod according to claim 12, wherein picture numbers are assigned tothe pictures stored in the memory; and in the judging step, thedetermined picture stored in the memory is read out, and the judgment ismade based on whether a picture number of the read-out picture is avalue obtained by subtracting the counted number from a picture numberof a picture to be decoded or more.
 15. The moving picture decodingmethod according to claim 12, wherein in the error processing step, theintra decoded picture is determined as a reference picture.
 16. Themoving picture decoding method according to claim 12, wherein in theerror processing step, a picture which has been decoded most recently isdetermined as a reference picture.
 17. The moving picture decodingmethod according to claim 12, further comprising an error correctingstep for reading out a picture which has been decoded most recently fromthe memory and outputting the read-out picture as it is.
 18. A movingpicture coding apparatus for coding a moving picture on a picture bypicture basis, the moving picture coding apparatus comprising: adetermination unit operable to determine, as a reference picture, one ofan intra coded picture and pictures which have been coded following theintra coded picture, from among a plurality of pictures stored in amemory; a coding unit operable to read out the determined referencepicture from the memory, calculate a differential picture signal that isa difference between the read-out reference picture and a picture to becoded, and code the calculated differential picture signal; and astorage unit operable to decode the coded differential picture signal,add the decoded differential picture signal with a picture signal of thereference picture, and store a resulting picture in the memory.
 19. Themoving picture coding apparatus according to claim 18, furthercomprising a counter unit operable to count the number of pictures whichhave been coded following the intra coded picture, wherein thedetermination unit determines the reference picture based on the numbercounted by the counter unit.
 20. A moving picture decoding apparatus fordecoding a moving picture on a picture by picture basis, the movingpicture decoding apparatus comprising: a decoding unit operable todecode an inputted coded picture signal; a determination unit operableto determine, as a reference picture, one of an intra decoded pictureand pictures which have been decoded following the intra decodedpicture, from among a plurality of pictures stored in a memory; and astorage unit operable to read out the determined reference picture fromthe memory, add a picture signal of the read-out reference picture witha decoded differential picture signal of a picture to be decoded, andoutput a resulting picture to the outside and store the resultingpicture in the memory.
 21. The moving picture decoding apparatusaccording to claim 20, further comprising a counter unit operable tocount the number of pictures which have been decoded following the intradecoded picture, wherein the determination unit determines the referencepicture based on the number counted by the counter unit.
 22. A recordingmedium which stores a program for coding a moving picture on a pictureby picture basis, wherein the program causes a computer to execute thesteps in the moving picture coding method according to claim
 1. 23. Arecording medium which stores a program for decoding a moving picture ona picture by picture basis, wherein the program causes a computer toexecute the steps in the moving picture decoding method according toclaim
 7. 24. A recording medium which stores stream data of a movingpicture which has been coded on a picture by picture basis, wherein thestream data has been coded by the moving picture coding method accordingto claim 1.